Dialog: Set SkipTaskbar/Pager states before showing window

This sets the states before showing the window and is a proper fix
for the linked bug.

Needs e4cea305ed2ba3c9f580bf9d16c59a1048af0e8a from qtbase - Qt 5.6.1

CCBUG: 332024
REVIEW: 127422

src/plasmaquick/dialog.cpp

@@ -1066,6 +1066,10 @@ void Dialog::showEvent(QShowEvent *event)
     if (d->backgroundHints != Dialog::NoBackground) {
         DialogShadows::self()->addWindow(this, d->frameSvgItem->enabledBorders());
     }
+
+    // TODO: Remove this call from other places once we can depend on Qt >= 5.6.1
+    KWindowSystem::setState(winId(), NET::SkipTaskbar | NET::SkipPager);
+
     QQuickWindow::showEvent(event);
 }